There is a significant ongoing effort to develop blends of different polymers so as to produce compositions having a desirable balance of properties and attributes, leading to enhanced products that are useful in a number of applications. Such enhancements include, but are not limited to, good tensile strength while still providing suitable mechanical recoverability (elastic recovery), impact strength, scratch resistance, and low compression set. As a result a wide variety of polymer blends are currently available, which depending on the miscibility of the components of the blend may be homogeneous (single phase) compositions or heterogeneous (multiple phase) compositions.
For example, U.S. Pat. No. 3,882,197 discloses homogeneous blends of stereoregular propylene/alpha-olefin copolymers, stereoregular propylene, and ethylene copolymer rubbers.
U.S. Pat. No. 3,888,949 discloses the synthesis of homogeneous blend compositions containing isotactic polypropylene and copolymers of propylene and an alpha-olefin, containing between 6-20 carbon atoms, which have improved elongation and tensile strength over either the copolymer or isotactic polypropylene. Copolymers of propylene and alpha-olefin are described wherein the alpha-olefin is hexene, octene or dodecene.
U.S. Pat. No. 6,635,715 discloses a composition formed by blending at least a first polymer component and a second polymer component, wherein the first polymer component comprises from about 2% to about 95% by weight of the blend and is formed from isotactic polypropylene having a melting point greater than about 110° C., wherein the second polymer component is produced by copolymerizing propylene and ethylene using a chiral metallocene catalyst system to produce a copolymer having a crystallinity from about 2% to about 65% from isotactic polypropylene sequences, a propylene content of from about 75% to about 90% by weight, a melting point of from 25 to 105° C., and wherein a glass transition temperature of said second polymer component is retained in the polymer blend.
None of the above disclosed blends use cross-linked semi-crystalline polymers, although thermoplastic blends with cross-linked polymers offer a number of advantages over uncross-linked blends, such as high impact strength, low compression set and scratch resistance. In fact, most thermoplastic blends with cross-linked polymers are currently heterogeneous (multiple phase) compositions.
Heterogeneous polymer blends generally comprise a matrix of a first polymer within which a second polymer is dispersed and, depending on the properties and the relative amounts of the first and second polymers, a wide variety of such polymer blends can be produced. Of particular interest are blends, also referred to as thermoplastic elastomers, in which the first polymer is a thermoplastic material, such as polypropylene, and the second polymer is an elastomeric material, such as an ethylene-propylene elastomer or an ethylene-propylene-diene (EPDM) rubber. The second polymer may or may not be cross-linked. Examples of such thermoplastic elastomers include polypropylene impact copolymers, thermoplastic olefins and thermoplastic vulcanizates. Unlike conventional vulcanized rubbers, thermoplastic elastomers can be processed and recycled like thermoplastic materials, yet have properties and performance similar to that of vulcanized rubber at service temperatures.
For example, U.S. Pat. No. 6,245,856 discloses a thermoplastic olefin composition comprising polypropylene, an ethylene-alpha olefin elastomer and a compatabilizer comprising an ethylene-propylene copolymer having a propylene content of greater than 80 weight percent. In the absence of the compatabilizer, the elastomer phase is said to be uneven with particles greater than 5 microns in size, whereas the addition of the compatabilizer is said to improve dispersion such that the elastomer phase has a particle size of about 1 micron. The elastomer phase of this heterogeneous polymer blend is not cross-linked.
One method of making polymer blends is by mixing two different polymers after they have been polymerized to achieve a target set of properties. However, this method is relatively expensive making it much more desirable to make blends by direct polymerization. Blending by direct polymerization typically uses multiple reactors in series, where the product from one reactor is fed to a second reactor having a different polymerizing environment, resulting in a final product that is an intimate mix of two different products.
For example, U.S. Pat. No. 6,207,756 describes a process for producing a blend of a continuous phase of a semi-crystalline plastic, such as polypropylene, and a discontinuous phase of an amorphous elastomer, such as a terpolymer of ethylene, a C3-C20 alpha olefin and a non-conjugated diene. The blends are produced in series reactors by producing a first polymer component in a first reactor, directing the effluent to a second reactor and producing the second polymer component in solution in the second reactor in the presence of the first polymeric component. U.S. Pat. No. 6,319,998 also discloses using series solution polymerizations to produce blends of ethylene copolymers. U.S. Pat. No. 6,770,714 discloses the use of parallel polymerizations to produce different polymeric components that are then blended through extrusion or using other conventional mixing equipment. One polymeric component is a propylene homopolymer or copolymer and the second polymeric component is an ethylene copolymer.
One particularly useful form of polymer blend is a thermoplastic vulcanizate (“TPV”), which comprises a thermoplastic resin matrix, such as polypropylene, within which are dispersed particles of a cured elastomeric material, such as an EPDM rubber. TPVs are normally produced by a process of “dynamic vulcanization”, which is a process of vulcanizing or cross-linking the elastomeric component during intimate, melt mixing of the blend with plasticizers (e.g. process oils), fillers, stabilizers, and a cross-linking system under high shear and above the melting point of the thermoplastic matrix. The mixing is typically done in a twin-screw extruder, to create a fine dispersion of the elastomeric material within the thermoplastic resin while the elastomeric material is cured. The levels of thermoplastic resin and plasticizer (oil) can be adjusted to produce grades having different profiles of hardness, rheology and engineering properties, although in general it is difficult to produce TPVs by dynamic vulcanization in which the content of the elastomeric phase is greater than 50 wt % of the overall polymer blend. Examples of dynamic vulcanization are described in the U.S. Pat. Nos. 4,130,535 and 4,311,628.
However, while dynamic vulcanization is effective in producing TPVs with a unique profile of properties, it is expensive and suffers from a number of disadvantages. Thus the production of quality product is technically challenging and specialized equipment is needed. Moreover, the process involves many steps, each one critical to the eventual quality of the final product. Forming the polymer blend normally involves separately comminuting bales of the elastomeric polymer (which is typically how EPDM rubber is commercially distributed), mechanically mixing it with the thermoplastic resin along with the processing oils, curatives, and other ingredients in a suitable high shear mixing device to comminute the rubber particles and cure them to generate cured rubber particles embedded in a continuous thermoplastic resin matrix. The cured rubber particles in the finished products have an averaged particle size of 1 to 10 microns. Careful injection of processing oil helps manage the rheological characteristics of the fluid in the reactive extruder (to minimize pressure buildup) as well as product properties such as hardness. Precise control over the size and distribution of the cross-linked elastomer particles is sought, as it affects properties such as elastic recovery (as measured through compression set). While the products produced with existing technology have many desirable properties, there are gaps in the overall properties profile. Some of these are the need for higher service temperatures, improved elastic recovery, softer products, higher tensile strength, easier processability, oil-free compositions, and/or colorless products.
An improved process for producing TPVs is disclosed in U.S. Pat. No. 6,388,016, incorporated herein in its entirety, in which a polymer blend is produced by solution polymerization in series reactors employing metallocene catalysts and the resultant blend is subjected to dynamic vulcanization. In particular, the process involves feeding a first set of monomers selected from ethylene and higher alpha-olefins, and a solvent, to a first continuous flow stirred tank reactor, adding a metallocene catalyst to the first reactor in an amount of 50 to 100 weight % of the total amount of catalyst added to all reactors, operating the first reactor to polymerize the monomers to produce an effluent containing a first polymer, feeding the effluent from the first reactor to a second continuous flow stirred tank reactor, feeding a second set of monomers selected from ethylene, higher alpha-olefins and non-conjugated dienes, and optionally additional solvent, to the second reactor, operating the second reactor to polymerize the second monomers to produce a second polymer containing diene, recovering the resulting first and second polymers and blending them with a curing agent under conditions of heat and shear sufficient to cause the blend to flow and to at least partially crosslink the diene-containing polymer and form a dispersion of cured diene-containing particles in a matrix of the first polymer. It will, however, be seen that this improved process still relies on dynamic vulcanization to cure the elastomeric component.
According to the present invention it has now been found that a homogeneous polymer blend comprising an at least partially cross-linked polymeric component can be produced by an in-reactor blending and cross-linking method.